Gas-Phase Condensation Reactions of Si x O y H z - Oxyanions with H2O

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2001-10, Vol.105 (42), p.9681-9688
• Main Authors: Groenewold, G. S, Scott, J. R, Gianotto, A. K, Hodges, B. D. M, Kessinger, G. F, Benson, M. T, Wright, J. B
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp010905e

Water was reacted with gas-phase oxyanions having the general composition Si x O y H z - that were formed and isolated in an ion trap-secondary ion mass spectrometer (IT-SIMS). The radical SiO2 •- reacted slowly with H2O to abstract HO•, forming SiO3H-, at a rate of 8 × 10-13 cm3 molecule-1 s-1, corresponding to an efficiency of about 0.03% compared with the theoretical collision rate constant (average dipole orientation). The product ion SiO3H- underwent a consecutive condensation reaction with H2O to form SiO4H3 - at a rate that was approximately 0.4−0.7% efficient. SiO4H3 - did not undergo further reaction with water. The multiple reaction pathways by which radical SiO3 •- reacted with H2O were kinetically modeled using a stochastic approach. SiO3 •- reacted with water by three parallel reaction pathways:  (1) abstraction of a radical H• to form SiO3H-, which then reacted with a second H2O to form SiO4H3 -; (2) abstraction of a radical OH• to form SiO4H-, which further reacted by consecutive H• abstractions to form SiO4H2 •- and then SiO4H3 -; and (3) condensation with H2O to form SiO4H2 •-, which subsequently abstracted a radical H• from a second H2O to form SiO4H3 -. In all of these reactions, the rate constants were determined to be very slow, as determined by both direct measurement and stochastic modeling. For comparison, the even electron ion Si2O5H- was also investigated:  it underwent condensation with H2O to form Si2O6H3 -, with a rate constant corresponding to 50% efficiency. The reactions were also modeled using ab initio calculations at the UB3LYP/6-311+G(2d,p) level. Addition of H2O to SiO3 •-, SiO3H-, and Si2O5H- was calculated to be approximately 42, 45, and 55 kcal mol-1 exothermic, respectively, and encountered low activation barriers. Modeling of SiO2 •- and SiO3 •- reactions with H2O failed to produce radical abstraction reaction pathways observed in the IT-SIMS, possibly indicating that alternative reaction mechanisms are operative.